1. Field of the Invention
This invention relates to an improved construction for X-ray fluorescence thickness measuring apparatus.
2. Description of the Prior Art
The utilization of X-ray fluorescence phenomena as a vehicle for the nondestructive measurement of the thickness of thin films on basal substrates of differing elemental composition has long been recognized, as exemplarly evidenced by U.S. Pat. Nos. 2,642,537 and 2,711,480. Articles by Zimmerman, "X-Ray Measurement of Plating Thickness" in American Jewelry Manufacturer, March 1961, p. 10 and "Industrial Applications of X-Ray Methods for Measuring Plating Thickness" in Quality Assurance, March 1963, p. 27 also evidence early commercial usage of such X-ray fluorescence phenomena in the measurement of plating thicknesses.
Fundamental to thickness measurement techniques employing X-ray fluorescence phenomena is the measurement of a quantum of fluorescent X-radiation generated by exposure of a calibration standard of known composition and film thickness to an incident X-ray beam and the comparison thereof with a similarly measured quantum of fluorescent X-radiation generated by exposure of a workpiece of like compositional character and unknown film thickness to an incident X-ray beam of like character. In general, the greater the quantum of detected fluorescent X-radiation, which a function of both intensity of the fluorescent X-radiation and the time permitted for detection thereof, the greater is the accuracy of measurement. As a practical matter, accuracy of measurement, because of essential statistical approach in detection and the desire to keep the time required for measurements to a minimum, is thus largely dependent upon the amount of detectable fluorescent X-radiation that is generated per unit of time. Such accuracy of measurement and the correlative reproducibility of the results obtained are, in turn, largely dependent upon the maintenance of a constancy of a number of interrelated operating parameters including the intensity of the primary X-ray beam, the diameter of incident primary X-ray beam relative to the area available for exposure, the positioning of the workpiece relative to the axis of the primary X-ray beam, the character and quality of the detector system and, as noted above, the time allotted for workpiece exposure.
The competitive pressures of recent years, coupled with increasingly restrictive product specifications and increases in the cost of commonly plated metals, e.g. gold, have contributed to a renewed focus on the utilization of X-ray fluorescence phenomena for thin film thickness measurement. Such renewed focus has in turn led to a need for higher degrees of measurement accuracy and reliability consistant with operational safety, increased measurement capacities, increased conveniences in usage and the minimization of operator participation in and control of the measurement process and the determination of the results thereof. Among some of the more recent patents that generally reflect such renewed focus are U.S. Pat. Nos. 4,162,528, 4,406,015, 4,534,049 and 4,172,223.